Electrical Networks MCQ Quiz - Objective Question with Answer for Electrical Networks - Download Free PDF

Explanation:

Alternating Current

Definition: Alternating Current (AC) is a type of electrical current in which the magnitude of the current changes continuously and its direction reverses periodically. This behavior contrasts with Direct Current (DC), where the current flows steadily in one direction.

Working Principle:

In AC, the flow of electrons alternates between moving forward and backward within the conductor. This alternating behavior is caused by the periodic reversal of the voltage polarity across the circuit. The frequency of the AC determines how often the current reverses direction per second. For example, in most countries, the standard frequency of AC is 50 Hz or 60 Hz, meaning the current reverses direction 50 or 60 times per second.

Characteristics:

• Magnitude: The magnitude of AC varies continuously with time, typically following a sinusoidal waveform. The voltage and current rise and fall in a smooth, cyclic manner.
• Direction: The direction of AC changes periodically, reversing back and forth, unlike DC, which flows in a single direction.

Advantages:

• Efficient Power Transmission: AC can be easily transformed to higher or lower voltages using transformers, making it ideal for transmitting electricity over long distances with minimal losses.
• Wide Application: AC is the standard form of electricity used in homes, offices, and industries because most electrical devices and appliances are designed to work with AC.
• Flexible Generation: AC can be generated using devices like alternators, which are simpler and more efficient than DC generators for large-scale electricity production.

Disadvantages:

• Complexity in Circuits: AC circuits require additional components like capacitors and inductors to manage voltage and current, which makes them slightly more complex than DC circuits.
• Safety Concerns: Due to its alternating nature, AC can be more dangerous than DC at high voltages, as the periodic reversal can cause more severe effects on the human body.

Applications:

• Domestic and industrial power supply.
• Operation of electrical appliances such as fans, refrigerators, and televisions.
• Powering large-scale machinery and equipment in factories.

Correct Option Analysis:

The correct option is:

Option 1: Changes continuously; changes periodically

This option accurately describes the behavior of an alternating current. The magnitude of AC changes continuously, typically in a sinusoidal pattern, and the direction of current flow reverses periodically, following the frequency of the AC system.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Does not change; remains the same

This description corresponds to Direct Current (DC), not Alternating Current (AC). In DC circuits, the current flows steadily in one direction, and its magnitude remains constant over time. Therefore, this option is incorrect for describing AC.

Option 3: Does not change; changes periodically

This option is inconsistent. If the magnitude of the current does not change, it cannot be categorized as AC since the defining characteristic of AC is the continuous variation in magnitude and periodic reversal in direction. This statement is contradictory and does not apply to AC.

Option 4: Changes continuously; remains the same

This option partially describes the behavior of AC but fails to account for the periodic reversal of direction. While the magnitude of AC does change continuously, its direction also reverses periodically. Hence, this option is incomplete and incorrect.

Conclusion:

Understanding the fundamental characteristics of Alternating Current (AC) is essential for differentiating it from Direct Current (DC). The correct option, as explained, accurately captures the behavior of AC, where the magnitude changes continuously and the direction reverses periodically. This unique property makes AC suitable for widespread use in power transmission, domestic applications, and industrial operations, despite its associated complexities and safety considerations.

Color Coding of Resistor

The value of the resistor is calculated as:

R = (First digit × 10 + Second digit) × 10^Multiplier

Calculation

Green = 5 (First digit)

Blue = 6 (Second digit)

Black = 0 (Multiplier = 10⁰ = 1)

Golden = ±5% (Tolerance)

R = (5 × 10 + 6) × 100

R = 56 Ω ± 5%

Fuse

• Fuse are designed to handle high fault currents and provide protection against short circuits and overloads in electrical systems. 
• A fuse is not a power-limiting device; instead, it is a current-limiting device designed to protect electrical circuits from excessive current.
• The fuse element inside a fuse is typically made from a high conductivity material, such as silver or copper. This element melts when excessive current flows through it, breaking the circuit.
• Fuse is consistent & it has the feature like if it has a high fault current then break time is low. Similarly, if the fault current is not high, then break time is long.
• In normal conditions, the flow of current through the fuse doesn’t provide sufficient energy to soften the element. If the huge current flows through the fuse then it melts the element of the fuse before the fault current achieves the climax.

A fuse protects circuits by breaking the connection when the current exceeds a safe limit, but it does not directly control or limit power consumption in normal operation. 

The correct answer is: 0.5 A

Key Points

• Maximum Current in AC Circuit:
  • The maximum current in a series RLC circuit is given by: I_max = V / R
  • Here, V = 6 V and I_max = 0.6 A
  • So, resistance R = V / I_max = 6 / 0.6 = 10 ohms
• Current through Coil with DC Cell:
  • Total resistance = resistance of coil + internal resistance of the cell
  • Given: resistance of coil = 10 ohms, internal resistance = 2 ohms
  • Total resistance = 10 + 2 = 12 ohms
  • Voltage from the cell = 6 V
  • So, current I = V / R_total = 6 / 12 = 0.5 A

Additional Information

• RLC Circuit in AC:
  • In AC, inductors and capacitors create reactance (opposition to current).
  • At resonance, inductive and capacitive reactances cancel out.
  • Only resistance (R) limits the current in that case.
• Ohm's Law:
  • Ohm's Law: I = V / R
  • This formula is used to find current when voltage and resistance are known.
  • It applies to both AC and DC circuits.

Magnetic Circuit

In a magnetic circuit, the force that tends to create magnetic flux is called MMF(magnetomotive force).

\(MMF=NI\)

where, N = No. of turns and I = Current

The SI unit of Magnetomotive force (MMF) is Ampere turn (AT).

Concept:

The energy is given by:

\(E=P\times t\)

\(E=(V\times I)\times t\)

where, I = Current

R = Resistance

t = Time

Calculation:

Given, V = 220 V

E = 1.5 kW

t = 1 hr

\(1.5\times 10^3=220\times I\times 1\)

I = 6.82 A

The analogy between Electric Circuit and Magnetic Circuit:

Concept:

The resistance of conductor changes when the temperature of that conductor changes.

New resistance is given by:

\({{R}_{t}}={{R}_{0}}\left( 1+\alpha \text{ }\!\!\Delta\!\!\text{ }T \right)\)

Where Rt = Final resistance or the resistance of the conductor after temperature changes

R0 = Initial resistance or the resistance of the conductor before temperature changes

α = temperature coefficient 

ΔT = final temperature – initial temperature 

Calculation:

Given that

\({{R}_{t}}={{R}_{0}}\left( 1+\alpha \text{ }\!\!\Delta\!\!\text{ }T \right)\)

\(R_{20}\)= 1 Ω

α = 0.005 Ω/°C

T1 = 20°C

\(R_{20}\) = \(R_0\) (1+0.005 * \(10^{-3}\)(\(20-T_0\))}

\(R_{0}\)= 1.1\(\Omega\)

Now resistance

\({{R}_{t}}={{R}_{0}}\left( 1+\alpha \text{ }\!\!\Delta\!\!\text{ }T \right)\)

∴ 2 = \(\frac{1}{1.1}\) [1 + 0.005 × (T₂ - 20)]

T₂ * \(5\times10^{-3}\) = \(\frac{11}{5}-1\)

T₂ =240° C 

Dependent sources depend on some other quantity and these can be classified into four types:

1) Voltage Controlled Voltage Source (VCVS)

2) Voltage Controlled Current Source (VCCS)

3) Current Controlled Voltage Source (CCVS)

4) Current Controlled Current Source (CCCS)

The correct answer is option 4): (0.02/° C)

Concept:

The temperature coefficient of resistance is generally defined as the change in electrical resistance of a substance with respect to per degree change in temperature

R_T = R₀ [1+ α (∆T)]

 The change in electrical resistance of any substance due to temperature depends mainly on three factors –

1. The value of resistance at an initial temperature.
2. The rise in temperature.
3. The temperature coefficient of resistance α.

Calculation:

\(\frac{\Delta R}{R_0} = α × \Delta T\)

∆T = 20° C

R_T = \(140\over 100\) R0

∆R = RT - R0

=  \(140\over 100\) R0 - R0

= ​\(40\over 100\)R0

\(\frac{\Delta R}{R_0} = α × \Delta T\)

\(40\over 100\) = α × 20

α  = 0.02/° C

Passive Element:

• The element which receives or absorbs energy and then either converts it into heat (R) or stored it in an electric (C) or magnetic (L) field is called passive element
• Do not need any form of electrical power to operate
• Not able to control the flow of charge
• Cannot amplify, oscillate, or generate an electrical signal
• Used for energy storage, discharge, oscillating, filtering and phase shifting applications
• Examples: Resistor, inductor, capacitor

Important:

Active Element:

• The elements that supply energy to the circuit is called an active element
• These have an ability to control the flow of charge
• Used for current control and voltage control applications
• Examples: Battery, voltage source, current source, diode

Ohm’s law: Ohm’s law states that at a constant temperature, the current through a conductor between two points is directly proportional to the voltage across the two points.

Voltage = Current × Resistance

V = I × R

V = voltage, I = current and R = resistance

The SI unit of resistance is ohms and is denoted by Ω.

It helps to calculate the power, efficiency, current, voltage, and resistance of an element of an electrical circuit.

Limitations of ohms law:

• Ohm’s law is not applicable to unilateral networks. Unilateral networks allow the current to flow in one direction. Such types of networks consist of elements like a diode, transistor, etc.
• Ohm’s law is also not applicable to non – linear elements. Non-linear elements are those which do not have current exactly proportional to the applied voltage that means the resistance value of those elements changes for different values of voltage and current. An example of a non-linear element is thyristor.
• Ohm’s law is also not applicable to vacuum tubes.

 ⇒ According to ohm's law, the Potential difference is directly proportional to the Current.

 ⇒ V œ I

 ⇒ V = IR ,where R is constant (Resistance)

⇒ V' =1/2 V then I' = 1/2 I

 ⇒ if the potential difference gets half then the current is also getting half.

Ohm’s law: Ohm’s law states that at a constant temperature, the current through a conductor between two points is directly proportional to the voltage across the two points.

Voltage = Current × Resistance

V = I × R

V = voltage, I = current and R = resistance

The SI unit of resistance is ohms and is denoted by Ω.

It helps to calculate the power, efficiency, current, voltage, and resistance of an element of an electrical circuit.

It an element follows the ohm’s law, then the element is known as a linear element.

Ex: Resistor

Limitations of ohms law:

• Ohm’s law is not applicable to unilateral networks. Unilateral networks allow the current to flow in one direction. Such types of networks consist of elements like a diode, transistor, etc.
• Ohm’s law is also not applicable to non – linear elements. Non-linear elements are those which do not have current exactly proportional to the applied voltage that means the resistance value of those elements’ changes for different values of voltage and current. An example of a non-linear element is thyristor.
• Ohm’s law is also not applicable to vacuum tubes.